The present invention pertains to sensors for sensing change in the amount of fluent material in a reservoir and, in particular, to a novel fluid sensor embodying novel electronic circuitry for providing a warning signal when the amount of fluid in a reservoir drops below a given level.
The prior art contains several types of fluid level sensors which provide a warning when the level of fluid in a reservoir drops below a predetermined level. In one type, a conductive probe is inserted into the reservoir, and electrical current is continuously conducted from the probe through electrically conductive fluid in the reservoir to ground. So long as the probe remains in contact with the fluid, a circuit is completed between the probe and ground, but when the fluid level drops to a point where the probe is no longer in contact with the fluid, then the circuit is opened. Appropriate circuitry can monitor the diminution of current through the probe to provide a warning signal indicating that the fluid level in the reservoir has dropped below a predetermined level.
Another type of prior fluid level sensor incorporates a temperature responsive device, such as thermistor, which is disposed within the reservoir at a given level and connected in an electric circit. Current is continuously passed through the thermistor, and so long as it remains immersed in fluid a certain amount of heat is dissipated through the fluid in the reservoir. Should the thermistor come out of the fluid then the heat transfer characteristics change to create a corresponding change in current flow through the thermistor which can be detected to provide a warning that the fluid level has dropped below the desired level.
Still another type of prior fluid level sensor continuously subjects a probe to an alternating current.
A common disadvantage of all these prior sensors is that they require a continuous current draw by the sensing element so long as they are operational.
The present invention is directed toward a novel fluid level sensor which overcomes the above discussed disadvantage of prior fluid level sensors by novel circuitry wherein the sensing element requires only intermittent current draw. The invention, rather than subjecting the sensing element to a continuous current draw, connects the sensing element in circuit with a capacitor thereby forming an RC circuit, then subjects this RC circuit to a transient condition, and finally monitors the transient reponse of the RC circuit. Where the sensing element is a conductive probe in contact with the electrically conductive fluid, such as conductive hydraulic brake fluid, the time constant of the RC circuit is such that the transient condition has a comparatively short time constant relative to the time constant which the circuit exhibits when the probe is not in contact with the fluid. The circuit monitors the nature of the transient response and provides a warning signal when the transient response is indicative of the probe being out of contact with the fluid. Where more than one reservoir is to be monitored, one species of the invention disclosed herein provides a novel arrangement whereby a single capacitor is time multiplexed with the probes in the reservoirs. The other species disclosed herein requires a capacitor for each probe and simultaneously subjects each capacitor/probe circuit to a transient condition at given sampling intervals. In thus subjecting each probe to an intermittent duty cycle type of operation, each species of the present invention includes an oscillator section which is believed novel by itself. The oscillator section establishes the duration of the sampling interval during which the fluid level in each reservoir is sampled and also the duration between successive sampling intervals. In the multiplex system, the oscillator section, in addition to establishing the duration of the sampling interval, is used in conjunction with additional circuitry to provide multiplexing. Each species of the invention is especially adapted for fabrication as a single, integrated circuit chip using C-Mos technology. In this regard, the invention provides a circuit capable of accomplishing its level sensing function with a comparatively small number of terminals being required to connect external components to the circuit chip. This is important in minimizing the cost of the system as well as contributing to the system reliability. By using C-Mos technology, the invention is particularly adapted for automotive application wherein only a single-ended power supply is available and the available electrical voltage may fluctuate over a considerable range.
The foregoing features and advantages of the invention, along with additional ones, will be seen in the ensuing description and claims which are to be taken in conjunction with the accompanying drawings.